When you are overweight and eat something fattening in public and someone says: "Are you sure you should be eating that?"
When you are quite thin, but some nosy scrotesque underweight old man sees that you have a big bag of M&M's and says: "It'll make ya fat."
When you are thin and someone who likes fat people tells you you're too thin. This is usually sexual harassment.

"I was trying to eat a doughnut one morning, but someone called the Calorie Cops. They were eating them too."
"I was proud of my thin new shape and this pervert who keeps trying to flirt with me said I should eat more. What is he, the Calorie Cops?"
Some people were making fun of fat people in line, and I said: "Shut up, you fvck!ng Calorie Cops!"